In lighting devices, it is often desired to make use of an optical system that controls the direction of light emitted from a light source. One such optical system that is frequently used is a collimator making use of total internal reflection (TIR), a so-called TIR collimator.
In FIG. 1 a prior art TIR collimator 1 in the form of a lens body 2 making use of total internal reflection is illustrated. As seen in FIG. 1, an extended light source 4 produces very wide cone angles for light rays 7 passing through a center portion 6 of the lens body 2, while in most regions light rays 8 that hits a TIR surface 5 produce quite narrow cone angles. Hence, a problem with the prior art TIR collimator 1 according to FIG. 1 is that cone angles emitted by different portions of the light exit surface 3 is highly non uniform. So when looking back into the TIR collimator, in a certain off axis angular range, no light will be emitted towards the observer from the TIR surface but only from the center portion; hence, a bright spot will be seen when observing a lighting device comprising the TIR collimator 1.
Hence, the emitted cone angle of light from the TIR collimator 1 typically is significantly larger for light passing through a central collimating portion 6 of the lens body 2 of the TIR collimator 1 than for light being subject to total internal reflection at the TIR surface 5. This implies that at an angle displaced from a main axis along which the light from the TIR collimator is directed, there will be a clearly observable distinction in luminance between light emitted from the central collimating portion 6 and light being subject to total internal reflection. Hence, the emitted light is observed at such an off-axis angle as a bright spot within a darker area. For spot lamps using several light sources and a TIR collimator array, the light emitted will be perceived as very spotty with the individual light sources being clearly distinguishable as bright spots with surrounding dark areas.
Accordingly, there is a need to improve light illumination from a TIR collimator such that the above mentioned spotty behavior is minimized.